Does High Jumping Cause Weightlessness?

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Discussion Overview

The discussion revolves around the phenomenon of weightlessness experienced by a high jumper as they clear the bar. Participants explore the physics behind this experience, considering different frames of reference and the implications of free fall in both classical physics and general relativity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why a high jumper experiences weightlessness while clearing the bar.
  • Another participant references Newton's second law (F=ma) in relation to the forces acting on the jumper.
  • A participant explains that during free fall, all parts of the jumper's body accelerate at the same rate, resulting in no net force and thus a state of weightlessness, similar to astronauts in orbit.
  • There are multiple interpretations of "weight," with one participant suggesting that weight is the apparent downward inertial force that must be countered to support a body in a specific frame of reference.
  • Different frames of reference are discussed, including a freely falling frame where the jumper is weightless, a frame tied to the Earth's surface where the jumper has normal weight, and a frame tied to the Earth's center where the jumper weighs slightly more due to Earth's rotation.
  • Another participant emphasizes that the jumper experiences weightlessness throughout the jump after losing contact with the ground, noting that in classical physics, the only force acting on a free-falling body is its weight.
  • In the context of general relativity, it is argued that the force acting on a body in free fall is zero as it moves through curved spacetime.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the nature of weightlessness and the forces acting on the jumper, indicating that multiple competing views remain without a consensus on the interpretation of weight and free fall.

Contextual Notes

Participants highlight the dependence on different frames of reference and the nuances in the definitions of weight, which may lead to varying interpretations of the jumper's experience.

Tulatalu
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Why a man clearing a bar in a high jump experiences weightlessness?
 
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F=ma even at the point of inflection.
 
The jumper is in free fall. All parts of his body are accelerated at an identical rate so that he does not experience any net force. Yes, (ignoring air resistance), he is weightless in the same sense that an astronaut in orbit about the Earth is weightless.

There are multiple meanings of the word "weight" which are subtly different. The one that I prefer to is that weight is the apparent downward inertial force that must be countered in order to support a body, motionless, in a particular frame of reference. The jumper's "weight" then depends on which frame of reference we adopt.

We can adopt a freely falling frame of reference in which the object is at rest, weightless.

We can adopt a frame of reference tied to the surface of the Earth in which the jumper weighs the normal amount.

We can adopt a frame of reference tied to the center of the Earth in which the jumper weighs a little bit more than what a spring scale would read due to the fact that the Earth is rotating.
 
Tulatalu said:
Why a man clearing a bar in a high jump experiences weightlessness?
The man experiences "weightlessness" not only as he clears the bar, but throughout the entire jump after he loses contact with the ground. All bodies in free fall are regarded as experiencing weightlessness. Ironically, within the framework of classical physics, bodies in free fall experiencing "weightlessness" have a single force acting on them, and it is their weight; go figure. Within the framework of general relativity, the picture is much more clear cut, because, in free fall, the force acting on the body is zero as it moves through curved spacetime.

Chet
 
Thanks everyone! :D
 

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